1. Field of the Invention
The present invention relates to an apparatus for and a method of measuring the image qualities of a color display unit such as a color television receiver, a television color cathode-ray tube, a display monitor (a color cathode-ray tube as a computer display monitor), a projection display unit, or the like.
2. Description of the Prior Art
Color display units such as color television receivers, television color cathode-ray tubes, display monitors (color cathode-ray tubes as computer display monitors), projection display units, etc. are subject to image quality factors including geometric raster distortions (image distortions), misconvergence (color shifts), color purity, color temperature, etc.
Image quality sensors for measuring image qualities may comprise photodiodes or one or more video cameras. Different image quality sensors may be switched over to measure different image quality factors.
Recently, there has been a demand for image quality measuring apparatus which have an image quality sensor comprising a single color video camera for measuring various image quality factors with high accuracy. Such image quality measuring apparatus should be easy to use, inexpensive to manufacture, and available in small quantities for a number of different design types.
One image quality measuring apparatus which meets the above demand employs a color video camera as an image quality sensor combined with a zoom lens. The color video camera captures the qualities of an entire image displayed on a color display unit being measured, and the image quality measuring apparatus measures image distortions, color shifts, and color purity among other image qualities.
Another image quality measuring apparatus which meets the above demand employs a monochromatic video camera as an image quality sensor combined with a fixed-focus lens. The monochromatic video camera captures the qualities of an entire image displayed on a color display unit being measured, and the image quality measuring apparatus measures image distortions among other image qualities.
However, the image quality measuring apparatus which employs a color video camera as an image quality sensor is disadvantageous in that since the primary optical filters in the color video camera have broad wavelength characteristics, crosstalk occurs between primary color signals, resulting in a reduction in the accuracy of color shift measurement. Since the amount of crosstalk varies depending on the wavelength of the three primaries radiated from the measured color display unit, if the crosstalk is to be canceled out, then it is necessary to take into account the emission characteristics of the color display unit, and the calibration process is tedious and time-consuming.
When the image quality measuring apparatus which employs a color video camera as an image quality sensor is used to measure image distortions, it is also necessary to determine which one of the primary color signals from the camera is to be used.
With the image quality measuring apparatus in which the color video camera is combined with a zoom lens, it is easy to match the field of view obtained by the camera to the dimensions of the measured color display unit. However, because the focal length of the zoom lens is not fixed, when image distortions are to be measured which require the position and dimensions of a displayed image to be measured, corrective calculations for calculating the absolute values of the dimensions are difficult to carry out, and as a result the accuracy of measurement of image distortions is lowered.
The image quality measuring apparatus in which the monochromatic video camera is combined with a fixed-focus lens offers merits for the measurement of image distortions, but needs to follow the process, given below, for measuring color shifts. Images displayed on the color display unit being measured are switched over for respective primary colors, and the positions of the respective primary color images are measured. The differences between the measured positions of the respective primary color images are then detected as color shifts. While the above process is being carried out to switch displayed images from one primary color to a next primary color and the position of the next primary color image is being measured, if the displayed primary color image suffers jitter or the image quality sensor or the color display unit being measured is mechanically vibrated for some reason, then the accuracy of measurement of the color shifts is reduced.